Question

How many moles of iron atoms are in 5.5 mol of Fe₂O₃?

Answer 1

To solve this you must know two things:
1.) What is the molar mass of Fe2O3.
2.) What fraction of that mass is contributed by Iron atoms.

1. Molar mass of Fe2O3 = (2 * 55.85g/mol) + (3 * 16.00g/mol) = 159.70 g/mol

2. Can be found by dividing the amount of mass contributed by iron atoms by total mass of the compound:
[ (2 * 55.85g/mol) / (159.70g/mol) ] * 100 = 0.69944
69.944% of the mass of each Fe2O3 formula unit is contributed by iron.

Now the no. of moles of Fe is given by, 5.5mol*0.6994 = 3.846 mol

The no. of moles of Fe in 5.5 mol of Fe2O3 = 3.846